Generally, in wireless communications, because an error sometimes occurs in a packet being transmitted through a transmission path for transmitting a packet, a technique of retransmitting from a transmitting device to a receiving device a packet in which an error occurred has been widely known. As a specific retransmission technique, there is an HARQ (Hybrid Automatic Repeat reQuest), for example. The HARQ is further divided into types such as CC (Chase Combining)-HARQ and IR (Increment Redundancy)-HARQ.
In the CC-HARQ, a transmitting device retransmits the same packet as that transmitted first, and a receiving device combines a first-received packet with a retransmitted packet, and performs decoding on the whole combined signal. Therefore, an SNR (Signal to Noise Ratio) cumulatively increases, and mutual information increases by the transmission of the packet. As a result, the precision of decoding in the receiving device improves, and thus errors of a packet can be reduced.
On the other hand, in the IR-HARQ, a transmitting device retransmits a packet containing a redundant bit different from that of a first-transmitted packet, and a receiving device performs decoding by using the redundant bits contained in a first-received packet and in the retransmitted packet, respectively. Therefore, redundant bits used for the decoding increase, an SNR cumulatively increases, and then mutual information increases. As a result, the precision of decoding in the receiving device improves, and thus errors of a packet can be reduced.
Further, with regard to these HARQs, there has been studied a technique in which a receiving device calculates a likelihood concerning each part of a packet, and when packet retransmission is necessary, a part having a calculated low likelihood is selected and retransmitted, and thus of a part having a high possibility of an error is retransmitted with priority.
However, when the HARQ is performed, a packet once transmitted is always retransmitted. Therefore, as compared with a case that there is no packet to be retransmitted, transmission of a packet containing a new information bit is delayed, and thus there is a limit in improving throughput. That is, when a transmitting device receives a NAK indicating that there is an error in a packet received by a receiving device, the transmitting device retransmits an already received packet prior to a packet containing a new information bit. Therefore, transmission of the packet containing a new information bit is delayed, and there is a limit in improvement of throughput.
Retransmission of a packet of the HARQ also has a problem that resources more than necessary are used to cancel a packet error. For example, in a relationship between an SNR and mutual information illustrated in FIG. 1, it is assumed that the mutual information needs to be equal to or larger than a threshold value Th for a receiving device to cancel an error of a packet by decoding. In this case, when an SNR of only a first-received packet is a (dB), the mutual information does not reach the threshold value Th, and therefore retransmission of a packet is requested to a transmitting device.
When packet retransmission is requested, the transmitting device retransmits an already transmitted packet, and therefore a cumulative SNR in the receiving device increases to c (dB). Because the mutual information exceeds the threshold value Th at this time, an error of the packet is cancelled by the retransmission. However, in practice, when an SNR becomes b (dB), the mutual information becomes equal to the threshold value Th, and therefore resources to change the SNR from b (dB) to c (dB) are used wastefully. In other words, energy is excessively consumed by the packet retransmission.    Patent Document Japanese Laid-open Patent Publication No. 2006-245912